Any stock installation of iOS or iPadOS includes a native Accessibility option called Reduce Transparency that, as the name suggests, reduces transparency in several interfaces found throughout the platform. Some of these interfaces include Today widgets, notification banners, and the Now Playing media player, to name a few.
Some people like using the Reduce Transparency option for the aesthetics alone; unfortunately, toggling this option applies to every aspect of iOS, which may not be preferred in all instances. Those looking for more fine-grained control over this feature may want to turn their attention to a newly released and free jailbreak tweak called Dusky by iOS developer P2KDev.